This invention relates generally to process refrigeration, and more particularly concerns the employment of a reaction turbine, or turbines, in such refrigeration, to improve efficiency.
A typical refrigeration system includes a compressor delivering pressurized refrigerant vapor to a condenser, a throttling valve receiving pressurized liquid refrigerant from the condenser and expanding same to produce colder liquid, and an evaporator wherein the cold liquid absorbs heat (from a body, room or fluid to be cooled) and evaporates for re-supply to the compressor. It has been proposed to replace the throttling valve (that expands the saturated refrigerant) with an expansion turbine. Extraction of shaft power will change the expansion at constant enthalpy that is characteristic for a throttling process to a nearly ideal isentropic expansion. The benefits derived by such an expansion are two-fold: the mass fraction of vapor produced as a result of the expansion is reduced when comparing the isentropic with the isenthalpic process. Secondly, power becomes available. The reduced vapor mass fraction means more liquid is available for evaporation cooling in the evaporator, and less vapor needs to be compressed.
One disadvantage of a conventional expansion turbine is the increased complexity of the machinery, which can reduce the reliability of the process. Typically, the entire two-phase refrigerant fluid mixture is run through the turbine nozzle and rotor passages. If droplets and vapor would follow the same paths (without droplet drift) the fluid could be considered pseudo-homogeneous with an average density considerably above that of the vapor. However, the concentrated masses of the droplets can be made to accelerate along curved paths only by substantial frictional drag forces exerted by the vapor, since pressure gradients are insufficient. In this regard, it is a good approximation to assume that the liquid droplets continue to move in a straight path in the initially assumed direction. Consequently, the droplets will impinge on the walls of curved nozzles and in the turbine buckets. The attending erosion and loss of efficiency make the application of a conventional expansion turbine questionable in mixtures where 90% of the mass is liquid. That conclusion is amplified when the volume ratio of the two phases is considered. Using a second stage ethylene expander as an example, the density ratio at the end of the expansion is 101.4; for a vapor mass fraction of 10% of the total mass the volume ratio of liquid to total volume becomes 1/12.3. Only 8.2% of the total volume flow is liquid. Since the turbine has to be dimensioned for handling the vapor and not only the liquid, equal velocities in both phases would spread the liquid (after impact) in a thin film over a large bucket surface. If the liquid path is long in relation to the hydraulic diameter of the liquid flow cross-section, the decline in liquid kinetic energy due to friction becomes large.